"Onigamiden" (2006), or "Legend of the Millenium Dragon", is an alright movie that has some slight issues, however, if you want to get as much out of it as possible, there are a few things you need to keep in mind before watching:
Onigamiden is currently rated 5,8 on IMDb. Even though the movie isn't perhaps one of the better animes out there, I do believe that this one is somewhat underrated. There are some fresh ideas and imagery used which, even though it doesn't save the movie, should justify a rating around 6,5 or so. Don't see this one with your mind made up it will be bad, but don't expect a masterpiece either.
The more knowledge you have about Japanese culture, the better. References are being made early to the beings known as "Oni", and characters recall and comments on different eras in the Japanese history, as well as old cities and villages. There are some points to gain here if one has a basic knowledge on those things.
Finally, the movie's plot is somewhat of a mess, and this is where its main problem lies. Objects which we understand have a big importance to the ongoing events are introduced to left and right without proper introduction, characters often do things while their intentions or motivations remains undisclosed, and the main protagonist has basically no character arc for the first 60-70 minutes. Some major loopholes can leave the viewer confused as well. I do not list this to discourage viewers, but rather to give you realistic expectations. It feels as though the movie was intended to be about two and a half hours, but an hour was edited out.
Overall, Onigamiden is an okay, care-free watch, but it needs not be prioritized too high on any's watchlist.
My personal grade: a puzzled 6/10. If you found this helpful, check my profile for more reviews.